阅读说明：本技术 一种高效的管线钢l245n生产工艺 (Efficient pipeline steel L245N production process ) 是由 隋鑫 张军 陆斌 柳婕 张帅 谷鑫 孙长玉 于 2021-07-27 设计创作，主要内容包括：本发明公开了一种高效的管线钢L245N生产工艺,本方法的成份设计更加经济适用,轧制方法采用正火轧制,通过高温轧制,轧后钢板在临界温度Ac3以上,模拟正火热处理状态的生产方式,起到细化晶粒,使组织均匀化的作用,通过减少亚共析钢中的铁素体含量,使珠光体含量增多并细化,从而提高钢的强度、硬度和韧性。此工艺不采用水冷工艺,避免了因为水冷工艺导致的头尾温差大的毛病,在提高性能合格率的同时提高了成材率。通过添加稀土元素,改善晶界,提高了钢板的低温冲击韧性。(The invention discloses a high-efficiency production process of pipeline steel L245N, the component design of the method is more economical and applicable, the rolling method adopts normalizing rolling, the rolled steel plate is rolled at a high temperature, the production mode of the normalizing heat treatment state is simulated at a critical temperature of Ac3, the effects of refining grains and homogenizing the structure are achieved, the content of pearlite is increased and refined by reducing the ferrite content in hypoeutectoid steel, and the strength, the hardness and the toughness of the steel are improved. The process does not adopt a water cooling process, avoids the defect of large head-tail temperature difference caused by the water cooling process, and improves the performance qualification rate and the yield at the same time. By adding rare earth elements, the crystal boundary is improved, and the low-temperature impact toughness of the steel plate is improved.)

1. A high-efficiency production process of pipeline steel L245N is characterized by comprising the following steps:

KR desulfurization and converter steelmaking:

KR desulfurization molten iron pretreatment: after pretreatment, the S content in the molten iron is less than or equal to 0.004 percent, the temperature of the molten iron is not less than 1300 ℃, and the slag is pneumatically pulled out in double ways, so that the exposed area of the molten iron is not less than 95 percent;

the proportion of the pretreated molten iron amount to the clean scrap steel is 5: 1-7: 1, the temperature of molten iron entering a furnace is 1300-1400 ℃, the S content in the molten iron is less than or equal to 0.04 percent, and active lime and high silicon dolomite must be added for slagging to ensure the alkalinity of slag; top and bottom combined blowing, wherein the carbon is drawn by the converter for one time, and the C-O balance at the blowing end point is ensured; the tapping hole does not scatter during tapping, the tapping time is not less than 5 minutes, slag washing operation is carried out during tapping, impurities are fully gathered and floated, the purity of molten steel is improved, and the end point temperature of the molten steel implements the current operation standard;

refining outside the furnace:

white slag is manufactured at a station for desulfurization, the slagging speed is high, the retention time is more than or equal to 15 minutes, and the final slag component is controlled to be CaO/SiO 2: 1-7.0: 1; the diameter of the bright ring does not exceed 1/3 of the diameter of the ladle during slagging; the deoxidation alloying of the molten steel requires that Ca treatment and soft blowing are carried out for more than or equal to 5 minutes after refining is completed, the slag surface is kept in a slightly fluctuating state during soft blowing, and the total content of O and S is required to be not more than 20ppm when the molten steel is discharged;

RH refining and rare earth treatment:

RH does not adjust components as much as possible, all the components are adjusted in LF, RH vacuum treatment is carried out for 15-17 minutes, rare earth alloy is added into a storage bin, the addition amount of the rare earth alloy is 17.5 kg/furnace, RH deep vacuum cycle time after the rare earth is added is guaranteed to be more than 15min, and argon soft blowing time is guaranteed to be more than 10 min;

continuous casting:

airtightness and the thickness of a tundish liquid surface covering agent are ensured among the reinforced bale, the tundish and the crystallizer, argon purging is performed on the reinforced tundish, air suction in the continuous casting process is avoided, and the N increase amount in the whole process is controlled within 5 ppm; the liquidus is 1520 ℃, the superheat degree of the tundish is controlled to be 23-33 ℃, the continuous casting pulling speed adopts the middle plate casting blank production pulling speed to execute steady-state casting, the thickness of the produced continuous casting blank is 250mm, and the protective casting is implemented in the whole continuous casting process;

heating:

the heating temperature is 1220-1260 ℃, the core temperature of the discharged plate blank is more than 1180 ℃, the heat preservation of the soaking section is ensured to be more than or equal to 40 minutes, and the thorough and uniform burning of the steel billet is ensured;

the rolling process comprises the following steps:

two-stage controlled rolling is adopted, the initial rolling temperature of one stage is more than 1100 ℃, and the reduction rate is 60-70%; the initial rolling temperature of the second stage is 880-920 ℃, the final rolling temperature is controlled to be 830-880 ℃, the reduction rate of each pass is more than or equal to 12%, and the cumulative reduction rate of the last three passes is not less than 40%;

shearing: and after cooling in a conventional air cooling mode, cutting in sections, sampling and warehousing.

2. The production process of the high-efficiency pipeline steel L245N as claimed in claim 1, wherein the steel comprises the following components by mass percent:

0.12% of C, 0.16% of Si, 1.10% of Mn, 0.017% of P, 0.002% of S, 0.032% of Al, 0.015% of Nb, 0.010% of Ti, 0.0003% of Ce, and the balance of Fe and inevitable impurities.

3. The production process of the high-efficiency pipeline steel L245N as claimed in claim 1, wherein the steel comprises the following components by mass percent:

0.11% of C, 0.15% of Si, 1.15% of Mn, 0.012% of P, 0.002% of S, 0.026% of Al, 0.016% of Nb, 0.012% of Ti, 0.0003% of Ce and the balance of Fe and inevitable impurities.

Technical Field

The invention relates to the field of normalizing rolling of medium plates, in particular to a high-efficiency production process of pipeline steel L245N.

Background

In recent years, with the gradual implementation of the major development strategy in the western part of China, the adjustment of energy structures and the gradual enhancement of environmental protection, the natural gas industry in China will have great development in this century, the national oil and gas pipeline network and the Asian oil and land bridge are planned to be tightly implemented by tightening and constructing and connecting the national and overseas oil and gas main arteries, the nation has listed 'West gas transport' in the national key infrastructure project, and the long-distance natural gas pipeline construction will enter the peak period in the coming years.

Pipeline transportation is the most economical and reasonable way for long-distance petroleum and natural gas transportation, and as the demand of petroleum and natural gas increases, pipeline steel develops rapidly. In order to improve the conveying benefit, reduce the energy consumption and reduce the investment, the long-distance pipeline is developed towards the direction of high-pressure and large-caliber conveying. Meanwhile, the method puts forward the target requirements of high strength, high toughness and high purity on the pipeline steel. The lower the carbon content of the current pipeline steel with higher grade, the higher the addition amount of the alloy element. This results in a corresponding increase in the production costs of the pipeline steel as the quality grades are increased. But for the relatively low-grade L245N-grade pipeline steel, the design of low-carbon components with the carbon content of 0.04-0.09% is adopted traditionally, the high range of the manganese content is 1.2-1.45%, and alloy elements such as niobium, vanadium, titanium and the like are added to cooperate with a TMCP rolling process to ensure the performance. The needle-shaped ferrite can be formed by adopting TMCP (thermal mechanical control processing) process production, and is beneficial to the properties of pipeline steel, such as drop hammer and the like, but has strict requirements on the rolling mill process: 1, the temperature of the steel plate is quickly reduced, the head-tail temperature difference is large, the head-tail performance is difficult to control, and the plate type of the steel plate is difficult to guarantee; 2, the tapping temperature is low, strict requirements on single-pass reduction of rough rolling are met, and the loss of a rough rolling mill is large.

Disclosure of Invention

In order to solve the technical problems, the invention aims to provide an efficient production process of the pipeline steel L245N, which can improve the one-time pass rate of the performance of the pipeline steel plate and improve the impact toughness.

According to the invention, aiming at L245N grade pipeline steel, after knowing the steel using requirements of users on the pipeline steel, the technical characteristic that the pipeline steel below the L390 grade does not need to be detected in the drop weight performance is regulated according to GB/T21237-2007 standard. The pipeline steel L245 process is improved from various aspects, and in order to solve the problems of low yield and poor qualification rate of the pipeline steel, the production process of the pipeline steel L245 is adjusted as follows:

the reasonable component design, the carbon content is 0.10-0.13 percent and the manganese content is controlled within the range of 1.10-1.30 percent, the steel burning temperature of the pipeline steel L245N is increased during rolling, the restriction of the rough rolling single-pass reduction is relaxed, the damage to equipment is reduced, and simultaneously the steel burning temperature of the pipeline steel L245N is the same as that of wind power and bridges, so that the tissue production is facilitated. The normalized rolling is the rolling at the temperature above the normalizing temperature, and the rolled steel plate is rolled at the temperature above the critical temperature Ac3 by high-temperature rolling to simulate the production mode of the normalized heat treatment state. The method has the effects of refining grains and homogenizing the structure, and increases and refines the pearlite content by reducing the ferrite content in the hypoeutectoid steel, thereby improving the strength, hardness and toughness of the steel; the process does not adopt a water cooling process, avoids the defect of large head-tail temperature difference caused by the water cooling process, and improves the performance qualification rate and the yield at the same time. Meanwhile, the rare earth cerium Ce obtained through research can improve the form of inclusions, purify grain boundaries, improve the strength of the grain boundaries, reduce the possibility of crack propagation through the penetration of defects, improve the impact toughness, refine the austenite grain boundaries, improve the L245N impact toughness of the pipeline steel, and reduce the ductile-brittle transition temperature of the steel by about 10 ℃ by adding the rare earth cerium Ce

In order to solve the technical problems, the invention adopts the following technical scheme:

the invention relates to a high-efficiency production process of pipeline steel L245N, which comprises the following steps:

KR desulfurization and converter steelmaking:

KR desulfurization molten iron pretreatment: after pretreatment, the S content in the molten iron is less than or equal to 0.004%, the temperature of the molten iron is not less than 1300 ℃, and the slag is drawn by adopting pneumatic double-drawing, so that the exposed area of the molten iron is ensured to be more than or equal to 95%.

The proportion of the pretreated molten iron amount to the clean scrap steel is 5: 1-7: 1, the temperature of molten iron entering a furnace is 1300-1400 ℃, the S content in the molten iron is less than or equal to 0.04 percent, and active lime and high silicon dolomite must be added for slagging to ensure the alkalinity of slag; top and bottom combined blowing, wherein the carbon is drawn by the converter for one time, and the C-O balance at the blowing end point is ensured; the tapping hole does not diffuse flow in the tapping process, the tapping time is not less than 5 minutes, slag washing operation is carried out in the tapping process, impurities are fully gathered and floated, the purity of molten steel is improved, and the current operation standard is executed at the end point temperature of the molten steel.

Refining outside the furnace:

white slag is manufactured at a station for desulfurization, the slagging speed is high, the retention time is more than or equal to 15 minutes, and the final slag component is controlled to be CaO/SiO 2: 1-7.0: 1; the diameter of a bright ring (a molten steel exposed area) does not exceed 1/3 of the diameter of a steel ladle during slag melting; the deoxidation alloying of the molten steel requires that Ca treatment is carried out for more than or equal to 5 minutes after refining is completed, the slag surface is kept in a slight fluctuation state during the soft blowing, and the total content of O and S is required to be not more than 20ppm when the molten steel is discharged.

RH refining and rare earth treatment:

RH does not adjust components as much as possible, all the components are adjusted in LF, RH vacuum treatment is carried out for 15-17 minutes, rare earth alloy is added into a storage bin, the addition amount of the rare earth alloy is 17.5 kg/furnace, RH deep vacuum cycle time after the rare earth is added is guaranteed to be more than 15min, and argon soft blowing time is guaranteed to be more than 10 min;

continuous casting:

airtightness and the thickness of a tundish liquid surface covering agent are ensured among the reinforced bale, the tundish and the crystallizer, argon purging is performed on the reinforced tundish, air suction in the continuous casting process is avoided, and the N increase amount in the whole process is controlled within 5 ppm; the liquidus is 1520 ℃, the superheat degree of the tundish is controlled to be 23-33 ℃, the continuous casting pulling speed adopts the middle plate casting blank production pulling speed to execute steady-state casting, the thickness of the produced continuous casting blank is 250mm, and the protective casting is implemented in the whole continuous casting process;

heating:

the heating temperature is 1220-1260 ℃, the core temperature of the discharged plate blank is more than 1180 ℃, the heat preservation of the soaking section is ensured to be more than or equal to 40 minutes, and the thorough and uniform burning of the steel billet is ensured;

the rolling process comprises the following steps:

two-stage controlled rolling is adopted, the initial rolling temperature of one stage is more than 1100 ℃, and the reduction rate is 60-70%; the initial rolling temperature of the second stage is 880-920 ℃, the final rolling temperature is controlled to be 830-880 ℃, the reduction rate of each pass is more than or equal to 12%, and the cumulative reduction rate of the last three passes is not less than 40%;

shearing: and after cooling in a conventional air cooling mode, cutting in sections, sampling and warehousing.

Further, the steel comprises the following components in percentage by mass:

0.12% of C, 0.16% of Si, 1.10% of Mn, 0.017% of P, 0.002% of S, 0.032% of Al, 0.015% of Nb, 0.010% of Ti, 0.0003% of Ce, and the balance of Fe and inevitable impurities.

Further, the steel comprises the following components in percentage by mass:

0.11% of C, 0.15% of Si, 1.15% of Mn, 0.012% of P, 0.002% of S, 0.026% of Al, 0.016% of Nb, 0.012% of Ti, 0.0003% of Ce and the balance of Fe and inevitable impurities.

The technical principle of the invention is as follows: the invention is different from the conventional pipeline steel in component design, the proper amount of added carbon element can improve the strength of the steel, reduce the yield ratio, slightly reduce the manganese content, prevent the manganese segregation in the pipeline steel casting blank from causing the anisotropy of the mechanical property of the steel plate and causing the reduction of the HIC resistance, improve the Ar3 temperature in the rolling process, ensure the ductile-brittle transition temperature of the steel plate to be in a proper range, add trace niobium, titanium and other alloy elements to improve the comprehensive property of the steel plate, and simultaneously reduce the surface defects of the steel plate. The rolling process adopts a two-stage normalizing rolling process, the steel burning temperature is increased, high-temperature, low-speed and high-pressure rolling is adopted, the dendritic crystal in the steel ingot is fully crushed, the larger the deformation degree is, the larger the density of a nucleation area and the driving force are increased, and the finer the crystal grain after repeated recrystallization is. In the first stage, multi-pass large-deformation rolling is carried out in an austenite recrystallization region, so that austenite is fully recrystallized, and the crushing and refining of grains are accelerated. And in the second stage, rolling is carried out in an austenite non-recrystallization region, the deformed austenite grains are elongated by plastic deformation, a deformation zone and carbonitride strain-induced precipitation of niobium and titanium trace elements are formed in the grains, the deformation zone becomes a ferrite grain nucleation point, and finally fine ferrite grains are obtained.

Compared with the prior art, the invention has the beneficial technical effects that:

compared with the prior art, the method has the advantages that the component design is more economical and applicable, the rolling method adopts normalizing rolling, the rolled steel plate is rolled at a high temperature, the production mode of the normalizing heat treatment state is simulated at a critical temperature Ac3, the effects of refining grains and homogenizing the structure are achieved, the content of pearlite is increased and refined by reducing the content of ferrite in the hypoeutectoid steel, and the strength, the hardness and the toughness of the steel are improved. The process does not adopt a water cooling process, avoids the defect of large head-tail temperature difference caused by the water cooling process, and improves the performance qualification rate and the yield at the same time. By adding rare earth elements, the crystal boundary is improved, and the low-temperature impact toughness of the steel plate is improved.

Drawings

The invention is further illustrated in the following description with reference to the drawings.

FIG. 1 shows the metallographic structure of the L245N pipeline steel 1/4 in example 1;

FIG. 2 shows the metallographic structure of the L245N pipeline steel 1/2 in example 2.

Detailed Description

Example 1:

the production process of the L245N-grade pipeline steel plate comprises the following components in percentage by mass: c is 0.12, Si is 0.16, Mn is 1.10, P is 0.017, S is 0.002, Al is 0.032, Nb is 0.015, Ti is 0.010, and Ce is 0.0003.

The main process steps and parameters are as follows:

molten iron fed into a furnace needs to be pretreated firstly, the sulfur content s of the treated molten iron is less than or equal to 0.010 percent, the temperature is more than or equal to 1250 ℃, slag needs to be removed completely before the molten iron is fed into the converter, the molten iron and scrap steel are added in the smelting process of the converter, and the ratio of the molten iron to the scrap steel is about 85 percent of the molten iron and about 15 percent of the scrap steel. C-T coordinated tapping is controlled at the end point of the converter, P is less than or equal to 0.012 percent, S is less than or equal to 0.015 percent, slag-stopping tapping is strictly performed, tapping time is 4-7 min, alloy is added when 1/5 steel is tapped, alloy is added after 2/5 steel is tapped, modifier and lime are added according to the oxygen content at the end point in the tapping process, oxygen is not less than 250m, the change of the loading amount, the end point C and the oxidability of molten steel is properly adjusted, and then the molten steel is conveyed to an LF refining furnace for refining operation.

Performing white slag making and deoxidation operations on molten steel by LF (ladle furnace) refining, ensuring that the content of elements such as oxygen, sulfur and the like in the steel is controlled at a lower level, quickly making white slag, ensuring that the white slag time is over 15min, and stabilizing the slag alkalinity; before LF refining is out of the station, the alloy is added to the target required range as much as possible, OT is less than or equal to 20ppm, N is less than or equal to 40ppm, and the S content is guaranteed to be less than 0.003%.

And after RH vacuum treatment is carried out for 4 minutes, rare earth alloy is added into a storage bin, 10% of cerium-iron alloy is added, the addition amount is 17.5 kg/furnace, the RH deep vacuum circulation time after the rare earth is added is ensured to be more than 15min, and the argon soft blowing time is ensured to be more than 10 min.

The liquidus line of the steel grade is 1520 ℃, the superheat degree of a tundish is controlled to be 23-33 ℃, the continuous casting drawing speed adopts the drawing speed of medium plate casting blank production to execute steady-state casting, the thickness of the produced continuous casting blank is 250mm, and the protective casting is implemented in the whole continuous casting process; the tightness and the thickness of the covering agent on the liquid surface of the tundish are ensured among the reinforced large ladle, the tundish and the crystallizer, and the argon purging of the tundish is reinforced to prevent the secondary oxidation and nitrogen increase of the molten steel. And (4) carrying out stack cooling on the off-line casting blank for more than 24 hours.

A heating system: the temperature of the heating section is not higher than 1260 ℃, the soaking temperature is 1220-1260 ℃, the heat preservation of the soaking section is ensured to be more than or equal to 40 minutes, and the thorough and uniform burning of the steel billet is ensured. The rolling process comprises the following steps: controlling rolling in two stages, wherein the initial rolling temperature in one stage is more than 1100 ℃, and the reduction rate is 60-70%; the initial rolling temperature of the second stage is 880-920 ℃, the final rolling temperature is controlled to be 830-880 ℃, the reduction rate of each pass is more than or equal to 12%, and the cumulative reduction rate of the last three passes is not less than 40%; and after conventional air cooling, performing segmented shearing, sampling and warehousing. The performance of the steel plate produced according to the technical scheme is shown in the table 1

TABLE 1 typical values of mechanical properties

The L245N pipeline steel plate produced according to the technical scheme is subjected to a Charpy impact test at the temperature of-10 ℃, and the low-temperature impact fracture is a ductile fracture. The metallographic microstructure is shown in fig. 1, and it can be seen from the metallographic microstructure that the microstructure is uniformly distributed ferrite + a small amount of pearlite, and the banded structure is rated 1A.

Example 2:

the production process of the L245N-grade pipeline steel plate comprises the following components in percentage by mass: c is 0.11, Si is 0.15, Mn is 1.15, P is 0.012, S is 0.002, Al is 0.026, Nb is 0.016, Ti is 0.012, Ce is 0.0003.

The main process steps and parameters are as follows:

molten iron fed into a furnace needs to be pretreated firstly, the sulfur content s of the treated molten iron is less than or equal to 0.010 percent, the temperature is more than or equal to 1250 ℃, slag needs to be removed completely before the molten iron is fed into the converter, the molten iron and scrap steel are added in the smelting process of the converter, and the ratio of the molten iron to the scrap steel is about 85 percent of the molten iron and about 15 percent of the scrap steel. C-T coordinated tapping is controlled at the end point of the converter, P is less than or equal to 0.012 percent, S is less than or equal to 0.015 percent, slag-stopping tapping is strictly performed, tapping time is 4-7 min, alloy is added when 1/5 steel is tapped, alloy is added after 2/5 steel is tapped, modifier and lime are added according to the oxygen content at the end point in the tapping process, oxygen is not less than 250m, the change of the loading amount, the end point C and the oxidability of molten steel is properly adjusted, and then the molten steel is conveyed to an LF refining furnace for refining operation.

Performing white slag making and deoxidation operations on molten steel by LF (ladle furnace) refining, ensuring that the content of elements such as oxygen, sulfur and the like in the steel is controlled at a lower level, quickly making white slag, ensuring that the white slag time is over 15min, and stabilizing the slag alkalinity; before LF refining is out of the station, the alloy is added to the target required range as much as possible, OT is less than or equal to 20ppm, N is less than or equal to 40ppm, and the S content is guaranteed to be less than 0.003%.

And after RH vacuum treatment is carried out for 4 minutes, rare earth alloy is added into a storage bin, 10% of cerium-iron alloy is added, the addition amount is 17.5 kg/furnace, the RH deep vacuum circulation time after the rare earth is added is ensured to be more than 15min, and the argon soft blowing time is ensured to be more than 10 min.

The liquidus line of the steel grade is 1520 ℃, the superheat degree of a tundish is controlled to be 23-33 ℃, the continuous casting drawing speed adopts the drawing speed of medium plate casting blank production to execute steady-state casting, the thickness of the produced continuous casting blank is 250mm, and the protective casting is implemented in the whole continuous casting process; the tightness and the thickness of the covering agent on the liquid surface of the tundish are ensured among the reinforced large ladle, the tundish and the crystallizer, and the argon purging of the tundish is reinforced to prevent the secondary oxidation and nitrogen increase of the molten steel. And (4) carrying out stack cooling on the off-line casting blank for more than 24 hours.

A heating system: the temperature of the heating section is not higher than 1260 ℃, the soaking temperature is 1220-1260 ℃, the heat preservation of the soaking section is ensured to be more than or equal to 40 minutes, and the thorough and uniform burning of the steel billet is ensured. The rolling process comprises the following steps: controlling rolling in two stages, wherein the initial rolling temperature in one stage is more than 1100 ℃, and the reduction rate is 60-70%; the initial rolling temperature of the second stage is 880-920 ℃, the final rolling temperature is controlled to be 830-880 ℃, the reduction rate of each pass is more than or equal to 12%, and the cumulative reduction rate of the last three passes is not less than 40%; and after conventional air cooling, performing segmented shearing, sampling and warehousing.

The performance of the steel plate produced according to the technical scheme is shown in the table 2:

TABLE 2 typical values of mechanical properties

TABLE 3 exemplary values of nonmetallic inclusions in L245N

The inclusion analysis of the L245N pipeline steel plate produced according to the technical scheme is shown in Table 3, the microstructure shown in figure 2 is ferrite plus pearlite, the crystal grains are fine, the structure is uniform, and the grain size is 8.5. The homogeneous fineness of the metallographic structure is in accordance with the mechanical properties of Table 2.

The embodiments show that the mechanical properties of the L245N pipeline steel plate produced by adopting the components and the process are far higher than the national standard, and the requirements of related industries on the low-temperature impact toughness of the pipeline steel plate are met.

The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.